Unless you were in low gear at the time, it's doubtful that the RD went into the spokes. It could be, as AL1943 pointed out, a stiff section of chain binding as it went through the cage and pushing the RD back, or it could not be a mechanical issue at all. Once in a while the wheels kick up a stick or other bit of road debris which jams into the chain and lower RD pulley.

The big variable here is the coincidence of it happening so soon after service. I scuba dive, and know that the greatest risk of a sudden regulator failure is during the first dive after service. I doubt there's any way to prove the exact sequence of events, and wish you good luck on the warranty. My fear is that the three parties will each blame the other.